S 2 Fluorescence Dynamics of meso -Aryl-Substituted Subporphyrins

Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins

Photosensitizers that display "unusual" emission from upper electronically excited states offer possibilities for initiating higher-energy processes than what the governing Kasha's rule postulates. Achieving conditions for dual fluorescence from multiple states of the same species requires molecular design and conditions that favorably tune the excited-state dynamics. Herein, we switch the position of the electron-donating NMe₂ group around the core of benzo[g]coumarins (BgCoum) and tune the electronic coupling and the charge-transfer character of the fluorescent excited states. For solvents with intermediate polarity, three of the four regioisomers exhibit fluorescence from two different excited states with bands that are well separated in the visible and the near-infrared spectral regions. Computational analysis, employing ab initio methods, reveals that the orientation of an ester on the pyrone ring produces two conformers responsible for the observed dual fluorescence. Studies with solid solvating media, which restricts the conformational degrees of freedom, concur with the computational findings. These results demonstrate how "seemingly inconsequential" auxiliary substituents, such as the esters on the pyrone coumarin rings, can have profound effects leading to "anti-Kasha" photophysical behavior important for molecular photonics, materials engineering, and solar-energy science.

Recent advances in subphthalocyanines and related subporphyrinoids

Subporphyrinoids constitute a class of extremely versatile and attractive compounds. Herein, a comprehensive review of the most recent advances in the fundamentals and applications of these cone-shaped aromatic macrocycles is presented.

A2B- and A3-Type Boron(III)Subchlorins Derived from meso-Diethoxycarbonyltripyrrane: Synthesis and Photophysical Exploration

The first direct fabrication of A₂B- and A₃-type B(III)subchlorins from meso-ethoxycarbonyl-substituted tripyrrane has been realized by condensation with appropriate acid chlorides (benzoyl chloride, butyryl chloride, and ethyl chlorooxoacetate). The aliphatic acid chloride-based annulation reaction is new to subporphyrinoid chemistry. The phenyl (6a)- or n-propyl (6b)-substituted derivatives could be oxidized to the corresponding B(III)subporphyrins upon refluxing with DDQ, whereas the triethoxycarbonyl moiety (6c) was found to be resistant to oxidation and exhibits the most red-shifted absorption (587 nm) and emission (604 nm). The study indicates that absorption and emission behaviors of the B(III)subchlorin can be tuned by the introduction of electron-rich or electron-deficient substituents at the meso-position. B(III)subchlorins 6a and 6c generate singlet oxygen efficiently (44 and 40%, respectively) and, thus, may find application as potential photosensitizers in photodynamic therapy (PDT).

Tracking Ultrafast Structural Dynamics in a Dual-Emission Anti-Kasha-Active Fluorophore Using Femtosecond Stimulated Raman Spectroscopy

The anti-Kasha process provides the possibility of using high-energy excited states to develop novel applications. Our previous research (Nature communications, 2020, 11, 793) has demonstrated a dual-emission anti-Kasha-active fluorophore for bioimaging application, which exhibits near-infrared emissions from the S₁ state and visible anti-Kasha emissions from the S₂ state. Here, we applied tunable blue-side femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy, assisted by quantum calculations, to reveal the anti-Kasha dual emission mechanism, in which the emergence of two fluorescing states is due to the retardation of internal conversion from the S₂ state to the S₁ state. It has been demonstrated that the facts of anti-Kasha high-energy emission are commonly attributed to a large energy gap between the two excited states, leading to a decrease in the internal conversion rate due to a poor Franck-Condon factor. In this study, analysis of the calculation and FSRS experimental results provide us further insight into the dual-emission anti-Kasha mechanism, where the observation of hydrogen out-of-plane Raman modes from FSRS suggested that, in addition to the energy-gap law, the initial photoinduced molecular conformational change plays a key role in influencing the rate of internal conversion.

De novo strategy with engineering anti-Kasha/Kasha fluorophores enables reliable ratiometric quantification of biomolecules

Fluorescence-based technologies have revolutionized in vivo monitoring of biomolecules. However, significant technical hurdles in both probe chemistry and complex cellular environments have limited the accuracy of quantifying these biomolecules. Herein, we report a generalizable engineering strategy for dual-emission anti-Kasha-active fluorophores, which combine an integrated fluorescein with chromene (IFC) building block with donor-π-acceptor structural modification. These fluorophores exhibit an invariant near-infrared Kasha emission from the S1 state, while their anti-Kasha emission from the S2 state at around 520 nm can be finely regulated via a spirolactone open/closed switch. We introduce bio-recognition moieties to IFC structures, and demonstrate ratiometric quantification of cysteine and glutathione in living cells and animals, using the ratio (S2/S1) with the S1 emission as a reliable internal reference signal. This de novo strategy of tuning anti-Kasha-active properties expands the in vivo ratiometric quantification toolbox for highly accurate analysis in both basic life science research and clinical applications.

Water-soluble chiral tetrazine derivatives: towards the application of circularly polarized luminescence from upper-excited states to photodynamic therapy

A new family of water-soluble chiral tetrazine derivatives 1 and 2 is reported. Spectroscopic studies reveal that the derivatives violate Kasha's rule and emit from their upper-excited states (S n , n > 1). The transition assignments are supported by time-dependent density functional theory calculations. More importantly, both chromophores exhibit anisotropy factors on the order of ∼10-3 to 10-4 for circular dichroism and circularly polarized luminescence (CPL) from upper-excited states. Additionally, the nonplanar geometry of the derivatives induces a significant yield of triplet excited states. Transient absorption spectroscopic measurements reveal high triplet quantum yields of ∼86% for 1 and ∼81% for 2. Through in vitro studies, we demonstrate that the derivatives can be used as photodynamic therapy (PDT) agents, providing a highly efficient form of cancer therapy. This study is the first demonstration of simple organic molecules with CPL from upper-excited states and efficient PDT.

Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin

This study revealed S₂ fluorescence from deprotonated meso-pentafluorophenyl-substituted Möbius aromatic [32]heptaphyrin(1.1.1.1.1.1.1) that was formed upon treatment of neutral antiaromatic [32]heptephyrin with tetrabutylammonium fluoride. Higher excited-state dynamics and emission were studied by fs-transient absorption spectroscopy and a broad-band fluorescence upconversion technique. This is the first S₂ fluorescence from chromophores with twisted Möbius topology, and the observation of S₂ fluorescence in the near-infrared region has been unprecedented. The higher excited-state dynamics of neutral and deprotonated [32]heptaphyrins were compared by ultrafast transient absorption spectroscopy to understand the S₂ fluorescence origin. In the antiaromatic [32]heptaphyrin, a fast time component of 65 fs was assigned as an internal conversion process from the S_B state to the S_Q state, which occurs prior to relaxation to the optically dark, lowest electronic state (S_D). Therefore, the S_Q state of the antiaromatic [32]heptaphyrin acts as a trap state intervening radiative transitions from the S_B state to the S₀ state. In deprotonated [32]heptaphyrin, the internal conversion from the S_B state to the S_Q state proceeds with a slower time constant of 150 fs for owing to its rigid structure, helping the observation of its S₂ fluorescence.

Recent Advances in Subporphyrins and Triphyrin Analogues: Contracted Porphyrins Comprising Three Pyrrole Rings

Subporphyrinato boron (subporphyrin) was elusive until the syntheses of tribenzosubporphine in 2006 and meso-aryl-substituted subporphyrin in 2007. These novel contracted analogues possess a 14π-electron conjugated system embedded in a bowl-shaped structure. They exhibit absorption and fluorescence in the UV/vis region and nonlinear optical properties due to their octupolar structures. The unique coordination geometry around the central boron atom in the structure of subporphyrin enabled investigation of rare boron species, such as borenium cations, boron hydrides, and boron peroxides. Along with the burgeoning development of the chemistry of subporphyrins, analogous triphyrin systems have also emerged. Their rich coordination chemistry as a result of their free-base structures, which are different from the boron-coordinating structure of subporphyrins, has been intensively investigated. On the basis of the unique structures and reactivities of subporphyrins and their related triphyrin analogues, supramolecular architectures and covalently linked multicomponent systems have also been actively pursued. This Review provides an overview of the development of subporphyrin and triphyrin chemistry in the past decade and future prospects in this field, which may inspire molecular design toward applications based on their unique properties.

Excited-state intramolecular proton-transfer reaction demonstrating anti-Kasha behavior

When excited with high energy quanta, specially designed 3-hydroxychromone derivatives demonstrate dramatic enhancement of the excited-state intramolecular proton transfer (ESIPT) reaction in obvious violation of Kasha's rule.

We report unusual photophysical properties observed on two newly designed 3-hydroxychromone derivatives exhibiting the excited-state intramolecular proton transfer (ESIPT) reaction. The efficiency of ESIPT reaction is greatly enhanced upon excitation with high energy quanta to S_n (n > 1) levels in low-polarity solvents. Based on detailed analyses of excitation and emission spectra as well as time-resolved emission kinetics we derive that conditions, in which this phenomenon contradicting Kasha's rule is observed, are quite different from that for observation of anti-Kasha emission.

Concerning correct and incorrect assignments of Soret (S2-S0) fluorescence in porphyrinoids: a short critical review

The relaxation dynamics of electronically excited porphyrinoids are often measured by steady-state and time-resolved fluorescence methods. The unusual occurrence of measurable fluorescence from an upper excited singlet state (often identified as the second electronically excited singlet state, S2) of some porphyrins has, in recent years, prompted a spate of mis-assignments of observed emission from other porphyrinoids excited in the near UV-violet regions of the spectrum. The criteria for correctly assigning fluorescence to a Soret excited state are reviewed. Questionable and mis-assigned reports are identified.

Photophysical properties of electron-deficient free-base corroles bearing meso-fluorophenyl substituents

The ultrafast photophysical behaviors of a series of meso-flurophenyl substituted electron-deficient free base corroles F₀C, F₅C, F₁₀C and F₁₅C in toluene have been investigated using femtosecond time resolved absorption spectroscopy and steady spectroscopies.

Hexaethylsubporphyrins: β-alkyl analogues in the subporphyrin family

Two new subporphyrins were synthesized for the first time from a β-substituted pyrrole i.e. 3,4-diethylpyrrole via pyridine-tri-N-(3,4-diethylpyrrolyl)borane as building blocks.

11,11,12,12-Tetracyano-4,5-pyrenoquinodimethanes: air-stable push-pull o-quinodimethanes with S2 fluorescence

The synthesis and properties of 11,11,12,12-tetracyano-4,5-pyrenoquinodimethanes (4,5-TCNPs), a new family of isolable and air-stable o-quinodimethanes, are reported. The ortho disposition of the dicyanomethane substituents strongly polarizes the pyrene framework to promote broad and intense intramolecular charge-transfer transitions. In addition, spectroscopic studies reveal that 4,5-TCNPs violate Kasha's rule and emit from the S2 level.

Unique ultrafast energy transfer in a series of phenylene-bridged subporphyrin–porphyrin hybrids

A series of phenylene-bridged subporphyrin–Zn(ii) porphyrin (SubP–ZnP) hybrid systems undergo extraordinarily fast excitation energy transfer (EET) processes from the SubP to the ZnP, aided by conjugative electronic elongation of the SubP to the phenylene-bridge.